Conventionally, as a porous material having a uniform pore size, a porous anodized alumina film is known. The porous anodized alumina film is a porous alumina film formed on an aluminum surface by anodizing aluminum in an acid electrolyte. As a characteristic feature, pores are self-aligned perpendicularly to the film surface, and the uniformity of the pore size is relatively satisfactory. For this reason, the porous anodized alumina film is expected to be used not only as a functional material represented by a filter but also as a starting structure for the form of various nanodevices.
The industrial applicability of such a porous material is considerably influenced by the regularity of the porous structure (pore shape and array). At this point a porous anodized alumina film formed by the conventional technique does not have a satisfactory regularity. More specifically, the porous anodized alumina film formed by the conventional technique has no independent pores perpendicular to the film surface. In addition, the interval between adjacent pores is not constant, and the pores are not round. As a result, the pore sizes are distributed in a wide range.
Straight and independent pores perpendicular to the film surface are not obtained due to the following formation mechanism of the porous structure of the porous anodized alumina film. At the start of anodizing, pores are formed at random, and some of them preferentially grow to form the porous structure. For this reason, at the early stage of anodizing, the porous structure has no regularity, and the pores are bent.